1. Field of the Invention
The present invention relates to an image-pickup optical system, such as for example a shooting or photographing system.
2. Description of the Related Art
A known image-pickup optical system having a long focal length includes, from the object side to the image side, a front lens component having positive refractive power and a rear lens component having negative refractive power. Such an optical system is known as a telephoto type optical system or, simply, a telephoto lens.
In a typical telephoto lens, as the focal length increases, longitudinal chromatic aberration and lateral chromatic aberration worsens in particular among the various types of chromatic aberration. In general, these types of chromatic aberration are corrected by combining a positive lens and a negative lens, where the positive lens is constructed of low-dispersion material, such as fluorite, having extraordinary partial dispersion and the negative lens is constructed of a high-dispersion material.
Extraordinary partial dispersion material efficiently corrects chromatic aberration but is difficult to process. The relative density of extraordinary partial dispersion material is greater than other low-dispersion materials not having an extraordinary partial dispersion characteristic. Thus, by using extraordinary partial dispersion material, the weight of the entire lens system increases.
For example, the relative density of fluorite is 3.18, and a commercial product FK01 known to be an extraordinary partial dispersion material similar to fluorite has a relative density of 3.63. In contrast, a commercial product FK5 that is a material whose extraordinary partial dispersion is small has a relative density of 2.46, and a commercial product BK7 has a relative density of 2.52.
Another known method for correcting chromatic aberration of an optical system is a method using a diffractive optical element (refer to “SPIE” (Vol. 1354, International Lens Design Conference (1990)) and Japanese Patent Laid-Open Nos. 6-324262 (corresponding to U.S. Pat. No. 5,790,321), 6-331887 (corresponding to U.S. Pat. No. 5,796,520), and 2000-258685 (corresponding to U.S. Pat. No. 6,381,079)).
The method uses the physical phenomenon in which chromatic aberration caused by a light beam having a specific wavelength is generated in opposite directions on the refracting surface and the diffracting surface in an optical system.
When a diffractive optical element is used as the lens closest to the object in a first lens unit of a telephoto lens, longitudinal chromatic aberration and lateral chromatic aberration of the optical system is very effectively corrected. However, when very intense light, such as sun light from the sun outside the angle of view, is incident on the diffractive optical element, flare and ghosts are generated. As a result, the entire image may be covered with a whitish haze.